The present invention provides a chemical heater for use in down-hole operations together with methods for operating such in oil/gas wells. The chemical heater comprises a heater body (1) housing a chemical reaction heat source material (2). The heater body (1) has a leading end that, in use, is deployed down-hole first. The heater is provided with ignition means (3), located in a heating zone proximal to the leading end of the heater body, for triggering a chemical reaction in the chemical reaction heat source material located in the heating zone (4, 4a). The heater is further provided with an actuator (5,6) housed within the heater body (1 ) and configured to urge the chemical reaction heat source material (2) towards the heating zone so as to generate a focused heating pattern.